This invention relates to an apparatus for controlling the synchronous operation of the spindles of a machine tool having two spindles. More particularly, the invention relates to a method of controlling the synchronous operation of the machine tool, which method is so adapted that the rotational speeds of the two spindles are made to coincide with excellent precision.
Machine tools having multiple spindle head units are widely employed as means for realizing shorter piece time when machining workpieces. Such machine tools are provided with two or more spindles for each tool rest, as seen, for example, in NC lathes, and the spindles are controlled by respective independent spindle motors to shorten the time needed to mount and dismount workpieces. Alternatively, the versatility with which workpieces can be machined is raised by employing spindle motors having different capabilities.
When the grip upon a workpiece is changed among a plurality of spindles, the same velocity command is applied to the spindle motors and, at the moment the velocities coincide, a transition is made to the next machining operation without stopping the rotating workpiece.
In such a conventional machine tool in which two spindles are arranged in opposition on the same axis and the tool rests are capable of performing machining on the workpiece irrespective of which spindle it is mounted on, it is necessary that the two spindle velocities coincide reliably when the workpiece is released by one spindle and grasped by the other. If this is not accomplished, there is the danger that the workpiece will be damaged or deformed by a chuck. If the spindle motors differ, however, it is difficult to bring the two rotational velocities into perfect agreement. In addition, rotational velocities fluctuate depending upon the load, even if the velocity commands are the same.
Accordingly, the present inventors have already filed an application (Japanese Patent Application No. 63-55765) for an invention in which a velocity error signal between the velocities of first and second spindles of a machine tool having these two spindles is added to a velocity command of the second spindle to correct the velocity command of the second spindle.
Synchronous operation in such case is achieved by applying the same velocity command to the spindles and limiting the torque of one of the spindles With this method, however, an offset develops between the two spindles if the velocity command is an analog signal, and there is the danger that the rpm's of the two spindles will shift away from each other, even if synchronism is temporarily achieved, owing to an error in a velocity feedback signal Consequently, when cutting is performed in a state where the same workpiece is grasped simultaneously by first and second spindles and the workpiece is separated into two portions by this machining, it is necessary to raise the precision of the synchronous operation and maintain the synchronized state. In other words, if an offset in synchronism develops, an excessive force acts upon the workpiece, thus making it impossible to perform highly precise machining.